It has long been recognized that retinal damage can result from the exposure of the retina to an intense light. The amount of damage is dependent upon the intensity of the light and the duration or period of time that the retina is exposed to the light. Recently, the medical community has begun to recognize instances of light induced retinal damage resulting from exposure to the intense light of an operating microscope illumination system. This problem is described in detail in an article by Michels and Sternberg, "Operating Microscope-Induced Retinal Phototoxicity; Pathophysiology, Clinical Manifestations and Prevention" published in the Survey of Ophthamology, Jan.-Feb., 1990, pages 237 to 252, which is incorporated by reference herein.
It is pointed out in the Michels and Sternberg article that the onset and severity of light-induced retinal damage is affected by the intensity of the light per unit area of retina exposed (watts per centimeter squared), the duration of exposure (minutes) and the wave length of the light source (Nanometers). Additionally, the fovea and the macula portion of the retina appear to be the most sensitive to retinal damage. This is a result of increased photo sensitivity of these acutely sensitive portions of the retina, the fact that damage to this area is much more readily noticed by the patient since the loss will occur in the prime viewing area, or some combination of these two factors.
During eye surgery, particularly during cataract surgery or interocular lens implantation, it is frequently necessary to directly illuminate the eye with an illumination source generally coaxially aligned with the microscope axis. The direct illumination axes are typically inclined relative to one another 3.degree. to 6.degree. and generally intersect one another at the focal plane of the microscope. Light from the microscope illumination system will pass through the eye and will be directed upon the macula portion of the patient's retina.
During a significant portion of any eye surgery procedure, it is also frequently not necessary to directly illuminate the eye. It is during these times that indirect illumination from an oblique light source can be utilized so as to minimize the duration of exposure of the macula to the microscope illumination system.
In order to minimize the likelihood of light induced retinal damage from an operating microscope illumination system, one manufacturer has proposed a counter which would monitor the cumulative exposure of the patient's retina to light. This is a product of a light intensity and the duration of exposure. Utilizing this monitoring technique, the ophthalmologist could appropriately limit the duration of the procedure in order to not overexpose the patient's retina to light. Others have proposed filtering out ultra violet and infrared portions of the illumination light spectrum in order to minimize the total energy absorbed by the retina of the patient's eye. Finally, by microscopes having dual illumination systems, a direct generally coaxial illumination system as well as a secondary illumination system having light directed upon the eye at an oblique angle relative to the microscope is being utilized. Microscopes having dual illumination systems are necessarily more complex and expensive. Zeiss and Topcon are two microscope manufacturers currently working on dual illumination system microscopes which utilize a pair of secondary light sources providing oblique illumination path inclined approximately 20.degree.from the microscope axis. The indirect illumination systems have their own separate light sources. During the surgical procedure when direct illumination is not necessary, the secondary illumination system is activated and the direct illumination system turned off.